1. Field of the Invention
The present invention relates generally to liquid cleanser compositions and, more particularly, to solubility enhancement of alkylbenzene sulfonates, such as low 2-phenyl alkylbenzene sulfonates in aqueous cleanser formulations by addition of ethylene oxide/propylene oxide block co-polymers to the formulation.
2. Description of Related Art
Linear alkylbenzene sulfonates ("LAS") are widely used surfactants in commercial cleanser products because of their effectiveness as detergents, ease of biodegradation, and relative low cost. Typically, linear alkylbenzene sulfonates are produced via sulfonation of linear alkylbenzene intermediates.
Linear alkylbenzene is typically manufactured on an industrial scale using one of two commercial processes which differ from one another primarily by virtue of the catalyst system employed. In this regard, one process employs an aluminum trichloride catalyst, while the other process uses a hydrogen fluoride catalyst. The two processes result in linear alkylbenzene products with different phenyl isomer distributions. For example, a typical phenyl isomer distribution for products of the aluminum trichloride process is about 30% 2-phenyl isomer and about 22% 3-phenyl isomer. In contrast, a typical phenyl isomer distribution for products of the hydrogen fluoride process is about 20% 2-phenyl isomer and about 20% 3-phenyl isomer, although reported values may differ. The product of the aluminum trichloride process, which is relatively high in 2-phenyl isomer content, is often referred to as "high 2-phenyl" linear alkylbenzene, whereas the product of the hydrogen fluoride process, which is relatively low in 2-phenyl isomer content, is often referred to as "low 2-phenyl" linear alkylbenzene.
The sulfonates of linear alkylbenzenes are known to exhibit different physical properties depending upon the position of the aromatic group on the alkyl chain. Therefore, high 2-phenyl linear alkylbenzene sulfonates have physical properties that differ from low 2-phenyl linear alkylbenzene sulfonates. For example, high 2-phenyl linear alkylbenzene sulfonates typically have a higher solubility in aqueous media than do low 2-phenyl linear alkylbenzene sulfonates. Furthermore, an aqueous solution comprising a high 2-phenyl linear alkylbenzene sulfonate may exhibit a higher viscosity than an aqueous solution comprising a low 2-phenyl linear alkylbenzene sulfonate. In cases where maximum solubility of linear alkylbenzene sulfonate in an aqueous detergent formulation is of concern, a product containing a relatively high percentage of compounds in which the aromatic substituent is in the 2 or 3 position and a correspondingly smaller percentage of isomers in which the aromatic substituent is positioned centrally with respect to the alkyl chain may be advantageous.
In the past, poor aqueous solubility has typically precluded the use of low 2-phenyl linear alkylbenzene sulfonates in detergent applications requiring a liquid formulation. For example, heavy duty liquid laundry detersive and liquid dishwashing detergent products have typically employed the more soluble high 2-phenyl linear alkylbenzene sulfonates as anionic surfactants. However, the relative high cost of high 2-phenyl surfactants often presents a disadvantage to cost-conscious detergent formulators and marketers. The higher cost of high 2-phenyl linear alkylbenzene sulfonates compared to low 2-phenyl surfactants stems from the greater expense associated with the aluminum trichloride process relative to the hydrogen fluoride process.
Attempts have been made to reduce the cost of liquid detergent formulations employing linear alkylbenzene sulfonates. Typically, these have included attempts to facilitate the use of the relatively less expensive low 2-phenyl linear alkyl benzene sulfonates. For example, one method typically employed for improving solubility of low 2-phenyl linear alkylbenzene sulfonates in liquid detergent formulations has involved the addition of hydrotropes, such as sodium xylene sulfonate. As used herein, the term "hydrotrope" is defined to be a compound that has the property of increasing the aqueous solubility of various slightly soluble organic chemicals. However, the cost advantage of low 2-phenyl formulations may be partially or completely offset by the cost of the relatively large amount of hydrotropes typically required to effect improved low 2-phenyl surfactant solubility without any accompanying improvement in the detergency characteristics of the formulation. Furthermore, addition of large amounts of hydrotropes to a detergent formulation may have the undesirable effect of lowering the viscosity of the detergent.